The present invention relates to a field memory self-refreshing device.
Because dynamic random access memory (DRAM) devices store data as electrical charges in memory cells, the accumulated electrical charge may be decreased by leakage current or other factors. Thus, it is necessary to periodically refresh all memory cells within a regular time period by increasing the charge and thus rewritting the aforementioned memory cells.
A field memory which is able to serially read/write, based on a reference clock, data of a single field in a television or video tape recorder which has the same memory cell structure as a DRAM memory cell. However, when the picture is scanned to display images based on the data in the field memory, the memory cells of the field memory are accessed serially based on a reference clock, and the result is the same as when the field memory is automatically refreshed. For example, when a single field of an NTSC color television signal is stored in a field memory, the refresh operation occurs every 16.6 ms because the field frequency of the NTSC signal is 60 Hz. However, when the memory cell access time is two to three times slower than the reference clock cycle, the refresh period becomes longer, possibly longer than the data hold time limit of the memory cell, and a self-refresh device becomes necessary.
Conventional field memory self-refresh devices are able to store the data for a single horizontal scan line in a single line memory block. Therefore, a refresh clock which executes multiple refresh operations while accessing the data for a single line (the same row address) is generated according to the reference clock and inputted to the refresh counter. Furthermore, a row address strobe signal for latching the refresh address is generated based on the aforementioned reference clock, and the refresh address from the aforementioned refresh counter is latched.
However, because the self-refresh device of the aforementioned conventional field memory generates the refresh clock and the row address strobe signal based on the aforementioned reference clock, the refresh clock and row address strobe signal generation operations will terminate if the reference clock stops, and the information in the memory cell will be destroyed.